It is well known that the viscosity of the thermoplastic material decreases with increases in temperature and pressure. In the field of hot-melt adhesives, small masses of thermoplastic material, solid at room temperature, are placed in a heating unit wherein walls of the unit transfer heat to the thermoplastic material melting it. In U.S. Pat. No. 3,964,645 thermoplastic material is introduced through the top of a hopper, heated in a lower receptacle which is fed by the hopper and then removed by a pump at the bottom of the receptacle. The success of this apparatus is partially due to the fact that there is adequate heat transfer from the walls of the receptacle to the thermoplastics to the extent that acceptable flow characteristics are obtained in the material. There are some very high viscosity thermoplastic materials, such as butyl, for which there is inadequate heat transfer from the walls of prior art melt units. For such substances, the melt time is unacceptably long, namely a few hours.
In Russian patent SU-578393 there is a teaching of electrical heating units which are hinged in rows to the walls of a chamber. These hinged heating units initially line the walls of the chamber and are intially in a flat position. Upon loading lumps of a material to be melted, the heaters are switched on and lifted, increasing heat transfer. Simultaneously, the melt is stirred by a propeller.
In the field of thermoplastic injection molding, it is known that granular thermoplastic materials can be melted more readily by agitating the granular material while heating. For example, in U.S. Pat. No. 2,174,319 agitation is applied to a heating receptacle by circulating spokes. Various types of paddles, vanes and blades are known in the prior art for the purpose of stirring. However, the problem for high viscosity thermoplastic materials is that such blades encounter too much resistance for effective operation.
An object of the invention was to devise an efficient heat transfer unit for melting and dispensing high viscosity hot-melt material, such as thermoplastics.